This invention relates to a device comprising an oxygen-sensitive element which has an oxygen ion conductive solid electrolyte layer, a reference electrode and a measurement electrode arranged to constitute a concentration cell and a current-supplying circuit to force a DC current to flow through the solid electrolyte layer to cause migration of oxygen ions through this layer from either one of the reference or measurement electrodes toward the other to thereby produce a reference oxygen partial pressure in the element while the measurement electrode is exposed to a gas subject to measurement.
In recent internal combustion engines and particularly in automotive engines, it has become popular to perform feedback control of the air/fuel ratio by utilizing an oxygen sensor installed in an exhaust passage as a device that provides an electrical feedback signal indicative of the air/fuel ratio of an air-fuel mixture actually supplied to the engine. Based on this feedback signal, a control circuit commands a fuel-supplying apparatus such as electronically controlled fuel injection valves to regulate the rate of fuel feed to the engine so as to correct deviations of the actual air/fuel ratio from an intended air/fuel ratio. Usually the oxygen sensor is an oxygen concentration cell utilizing an oxygen ion conductive solid electrolyte, such as zirconia stabilized with calcia or yttria, and there is the need for establishing a reference oxygen partial pressure to enable the oxygen sensor to generate an electromotive force in dependence upon the difference between an oxygen partial pressure in a gas subject to measurement and the reference oxygen partial pressure. It has been a usual practice to utilize either air or a mixture of a certain metal such as nickel and its oxide as the source of oxygen to establish the reference oxygen partial pressure.
U.S. Pat. No. 4,207,159 discloses a new type of oxygen-sensing device which is essentially a combination of an oxygen-sensitive element having a microscopically porous layer of an oxygen ion conductive solid electrolyte with reference and measurement electrode layers formed thereon, a DC power source to force a constant current to flow through the solid electrolyte layer between the two electrode layers and a voltage measuring means to measure an output voltage developed between the two electrode layers of the oxygen-sensitive element. In this device, a reference oxygen partial pressure is established in the oxygen-sensitive element, without using any extra oxygen source material, by a balance between the migration of oxygen ions through the solid electrolyte laser in a definite direction caused by the flow of the aforementioned current and diffusion of oxygen molecules through the porous solid electrolyte layer. Wide applications of this device is expected primarily because of the simplicity of the construction of the oxygen-sensitive element as the result of eliminating the need to use an oxygen source material and the desirability of determining the magnitude of the reference oxygen partial pressure by appropriately determining the intensity of the constant current supplied to the element.